1. Field of the invention
This invention relates broadly to endoscopes and laparoscopes. More particularly, this invention relates to relay lens systems for endoscopes and laparoscopes.
2. State of the Art
Endoscopes are optical systems which are well known in the art for allowing the optical viewing of otherwise inaccessible areas within the human body without the use of excessive surgery. Besides minimizing the invasiveness of the surgery, endoscopes provide the advantage that, depending on the optical resolution of the endoscope, the taking of biopsy samples for later laboratory analysis may be unnecessary where direct endoscopic optical diagnosis is sufficient. Laparoscopes are specialized endoscopes which are primarily used to examine the peritoneal cavity (pertaining to the abdominal and pelvic cavities) of a patient.
Structurally, endoscopes generally include an airtight and waterproof elongated tube having a distal end placed within the body cavity of the patient to be examined, and a proximal end for optical viewing by the physician. The elongated tube is usually comprised of three significant optical parts: the objective lens; the relay system; and the eyepiece. The objective lens is located at the distal end of the tube, and forms a first inverted image of the observed area. The function of the relay lens system is to take the image formed by the objective lens, and to refocus and reinvert the first image several times in order to form a final upright image at the proximal end of the tube. The final upright image is then observed through the eyepiece by an observer (e.g., a surgeon).
A typical endoscopic relay lens system is made of seven identical and often symmetrical compound lenses, four of which act as field lenses and three of which act as imaging lenses. The field lenses are situated at the image planes of the relay system and serve to receive and keep together the image rays which are then refocused by the imaging lens into a new image. More specifically, in the seven relay lens configuration, the first inverted image focused at the distal end by the objective lens is reimaged twice before being formed into a final upright image at the proximal end for viewing through the eyepiece. Each lens component of the relay lens system is further typically made of at least two or more elements to correct for inherent chromatic and geometric (spherical, field curvature, astigmatic, coma) aberrations.
A problem that arises in designing adequate relay lens systems is that as the first inverted image is transmitted by the relay system from the distal end to the eyepiece, it loses brightness, contrast and definition.
The brightness of the image is related to the ratio of the focal length and the diameter of the lens through which the image is being focused. This ratio is also referred to as the f/number. The smaller the f/number, the brighter the image transmitted by the lens. It is therefore preferable in an endoscopic relay lens system to have an f/number as low as possible for the system so that as much image brightness as possible is transmitted from the objective lens to the eyepiece. While the system f/number more or less defines brightness, brightness is also lost due to absorption of energy by the lens media as well as well as unwanted reflection losses at lens element interfaces.
Contrast is degraded because of the scattered light from lens element media imperfections and the unwanted reflected light from lens interfaces contribute to a non-imaged background light which dilutes the image.
Definition of the image is lost due to abberrations of the optical system which results in a blurring of the image sharpness or resolution. This blurring increases as the image passes through successive lens elements.
The above mentioned deficiencies are at least partially addressed in U.S. Pat. No. 3,257,902 to Hopkins which discloses the use of rod-like glass lenses for the major part of the length of the relay lens system in an endoscope. These rod lenses exhibit lower effective f/numbers than conventional lenses, and, as a result, will transmit a brighter image than a conventional system over the same distance of the relay system; or conversely, an equally bright image will be transmitted by a rod lens system over a greater distance of the relay system than that transmitted by a conventional lens system. In addition, the use of rod lenses may allow the use of fewer reimaging and field lens elements resulting in fewer interfaces on which the light rays will be reflected and refracted in the system. However, the arrangement disclosed in the Hopkins patent is relatively complex and requires difficult optical fabrication of the glass rod lens elements. This leads to corresponding difficulties in high volume manufacturing. In addition, the Hopkins design fails to disclose whether its components are sufficiently temperature resistant such that they can be used in an autoclavable endoscope.
The shortcomings of the endoscope disclosed in the Hopkins patent were partially addressed in U.S. Pat. No. 4,784,118 to Fantone et al. which discloses a relay lens system for a disposable endoscope including inexpensive and easy to manufacture polymeric rod lenses. In spite of the improvements in the cost and method of manufacture of the polymeric rod lenses of the relay lens system disclosed by Fantone, that system still suffers from various astigmatic and chromatic optical aberrations. The design of the rod lens system in Fantone incorporates identical polymeric rod lenses having no additional lens elements for the correction of color or other aberrations inherent in an optical system. The image transmitted still lacks adequate brightness and clarity. The system disclosed in Fantone also fails to disclose an inexpensive autoclavable endoscope design with temperature resistant components, and in fact, the patent proposes lenses which would "melt" at autoclave temperatures.
Another problem that arises in the design of adequate relay lens systems is that inherent chromatic and geometric aberrations associated with lenses are compounded in an optical transmission system containing multiple field and imaging lenses. Although it is known in the art to use achromatic, aspheric and multiple lens configurations to correct these aberrations, it is more difficult to do so in longer optical transmission systems for the above mentioned reasons. The Fantone patent does not adequately correct for the chromatic and axial aberrations and the significant resulting image quality loss suffered during transmission of the image from the objective lens to the eyepiece.